Calcium aluminates are one of the major components of aluminous cements that are frequently used for making binders, concretes or mortars (construction chemistry mortars, refractory mortars, etc.).
There are two main aluminous cement classes to be particularly distinguished from each other: crystalline calcium aluminate cements characterized both by their chemical composition and their crystalline phases (mineral phases), and amorphous calcium aluminate cements characterized by their chemical composition.
In fact, calcium aluminates are in part amorphous or in part crystalline, and are classified among one class or the other, depending on the rates of amorphous phases and crystalline phases they include.
Calcium aluminates having mostly crystalline phases are subsequently referred to as “crystalline calcium aluminate”, and calcium aluminates having mostly amorphous phases are subsequently referred to as “amorphous calcium aluminate”.
Here, we will especially focus on compositions comprising amorphous calcium aluminates. These are for the most part composed of calcium oxide or lime CaO (also noted C in cement notation), alumina Al2O3 (noted A in cement notation), and optionally of silica SiO2, iron oxide Fe2O3, or other impurities commonly included in the raw materials.
It is known that amorphous calcium aluminates may be characterized by their molar or their weight ratio of lime (C) to alumina (A), more frequently referred to as the C/A ratio (cement works abbreviation).
Indeed, the properties of the amorphous calcium aluminates, as well as the applications resulting therefrom highly depend on the C/A molar ratio.
For example, binders containing amorphous calcium aluminates which C/A molar ratio is higher than 1.8 have a very fast setting time, that is to say of about a few minutes at most. On the contrary, those which C/A molar ratio is lower than 1.8 have a slower setting time, which may extend to a couple of hours.
As an example, an amorphous calcium aluminate with a C/A molar ratio lower than 1.8 will be chosen when a binder is expected, with a relatively long implementation timeline and some workability over time (auto-leveling soil resurfacing or precast work); or an amorphous calcium aluminate with a C/A molar ratio ranging from 1.5 to 1 will be chosen when a binder is expected, for use in the production of a screed or a flooring adhesive (setting time between 2 and 4 hours).
On the contrary, when an ultra-fast setting binder is expected, for use in the production of mortar or concrete in a wet environment, amorphous calcium aluminate will be used with a C/A molar ratio ranging from 1.8 to 2 (setting time of from 10 to 20 minutes). Likewise, for making a shotcrete or an anchoring capsule for which a setting time of from about a few seconds to 10 minutes is needed, an amorphous calcium aluminate will be chosen with a C/A molar ratio ranging from 2 to 2.2.
Thus, for each intended application, it is required to accurately determine and to optimize the C/A molar ratio of the amorphous calcium aluminate used and conversely, precisely determining the C/A molar ratio of an amorphous calcium aluminate makes it possible to choose the applications suitable for it. The corollary to this is that, once the application is determined, the molar ratio value of the amorphous calcium aluminate used should remain as close as possible to the ideal ratio required by the application, typically in the range of ±0.1, ideally in the range of ±0.05, or ±0.02.
Today, the amorphous calcium aluminates are therefore produced so as to have a predetermined lime to alumina molar ratio depending on the intended application.
In practice, such amorphous calcium aluminates having a predetermined C/A molar ratio may be currently obtained through a so-called chemical process, for example, through a melting process followed by a fast cooling-down. Document DE3610586 gives an example of such an amorphous calcium aluminate.
Obtaining by a chemical reaction this amorphous calcium aluminate with the C/A molar ratio predetermined is achieved through given time and temperature conditions, together with reactants selected and combined in suitable proportions.
In particular, this amorphous calcium aluminate may be currently obtained by a melting process. This melting process consists in heating in a vertical melting furnace and at very elevated temperatures (1300° C.-2300° C.) a suitable amount of limestone blocks (CaCO3) and a suitable amount of monohydrate bauxite (a mineral rock that is rich in alumina and that contains iron and silica in variable amounts) for a time period enabling the complete melting of these raw materials (from around 2 to 10 hours). After this step, a liquid mass is recovered through a tap hole located in the lower part of the furnace. The liquid mass, which has in particular a temperature ranging from 1300° C. to 1600° C. is then suddenly drastically cooled-down to a temperature below its crystallization temperature (typically at most 1200° C., more conventionally lower than 1000° C.). Once cooled down, the product called clinker, is thereafter ground to form an amorphous calcium aluminate in the form of a powder, also called amorphous calcium aluminate cement.
However, this process of production requires restrictive and expensive operating conditions, both regards the energetic and the time requirements. Indeed, the use of very elevated temperatures, ranging from 1300° C. to 1600° C., for a substantial time period are needed for making this chemically-induced amorphous calcium aluminate.
In addition, raw materials such as bauxite blocks comprise, to varying degrees, oxides (iron oxide, silica) which may be detrimental to the reproducibility of the thus formed amorphous calcium aluminate and also to the expected C/A molar ratio specific determination.
Furthermore, since the intended application of each amorphous calcium aluminate depends on its C/A molar ratio, it is appropriate to produce separately a large number of different amorphous calcium aluminates, each having a C/A molar ratio suitable for a particular application, which increases the industrial complexity.
In particular, for each targeted lime to alumina molar ratio (C/A), it is necessary to optimize the experimental conditions, especially, the choice of the raw materials and their respective proportions, the curing temperature (a high alumina rate requires a higher curing temperature), the curing time, and the final cooling down time.
Furthermore, the crystalline calcium aluminates are known for a long time and are produced by a melting process followed by a slow cooling down, or by sintering.
They are for example, described by Kopanda et al., in the publication «Production Processes, Properties and Applications for Calcium Aluminate Cements», Alumina Chemicals Science and Technology handbook, American Ceramic Society (1990), pp 171-181. Among the main crystalline phases reported by Kopanda et al., those who are most commonly upgraded in the industrial applications are monocalcium aluminate (CaO—Al2O3, or CA in cement notation), dodeca-calcium hepta-aluminate ((CaO)12—(Al2O3)7, or C12A7 in cement notation) or monocalcium di-aluminate ((CaO)—(Al2O3)2, or CA2 in cement notation).
Different products that contain crystalline calcium aluminates are commercially available and it is known to classify these products by their alumina rate, i.e. by the alumina weight they contain, based on the total weight of the dry product: about 40% (in the Ciment Fondu® of Kerneos Company), 50% (in the Secar®51 of Kerneos Company), or 70% (in the Secar®71 of Kerneos Company).
It is also known that the dissolution rate of the crystalline calcium aluminates mixed with water is highly dependent on the nature and the amounts of each crystalline phase comprised in the crystalline calcium aluminate.
For example, the CA crystalline phase has a dissolution rate suitable for applications requiring a control of the open time, i.e. a control of the time during which it is possible to process the crystalline calcium aluminate once it has been mixed with water. Examples include Secar®71, Secar®51 cements and Ciment Fondu® cited above.
Conversely, crystalline calcium aluminates containing more lime, especially more crystalline phase C12A7, can be used for certain applications requiring hydration (or a water dissolution) as quickly as possible (e.g shot concrete). An example includes the product marketed in the past under the trademark Shotax® by Lafarge Fondu International Company.
Thus, it is known to combine various crystalline calcium aluminates to optimize the dissolution rate of a crystalline calcium aluminate.
However, it is found that mixing in water two crystalline calcium aluminates, each having a known overall chemical composition, i.e. a known C/A molar ratio, does not predict the final C/A molar ratio of the dissolved crystalline calcium aluminate resulting from this mixing.
Besides, mere knowledge of the chemical composition of a crystalline calcium aluminate does not predict its behavior, so that an increase of a few percentage points of C12A7 can result in a significant decrease in the open time of a mortar or lead to an immediate setting of this mortar.
For example, the man skilled in the art knows that the mixture of two crystalline calcium aluminates, ground to the same fineness (or Blaine's specific surface area), one containing mainly the crystalline phase C12A7 (such as Shotax®) with a C/A molar ratio that is equal to 1.7, and the other containing the crystalline phases CA and CA2 (such as Secar®71) with a C/A molar ratio that is equal to 0.64, the mixing being conducted in suitable proportions to provide a C/A molar ratio of 1 for the resulting crystalline calcium aluminate, results in a flash setting (i.e. of about one minute) when it is prepared in simple mortar (preparation and composition according to Standard EN 196-1, but with 500 g of cement, 1350 g of sand and 200 g of water). Yet, the behavior associated with this crystalline calcium aluminate mixture is very different from the behavior associated with a crystalline calcium aluminate of CA crystalline phase, with a C/A molar ratio close to 1, directly obtained by a melting or sintering process, which has in the same test conditions a setting time of about three hours.
Accordingly, there is no general method for designing a crystalline calcium aluminate mixture which reactivity would be controlled or could be predetermined.
Furthermore, it is also known to mix a crystalline calcium aluminate and an amorphous calcium aluminate in order to obtain a final calcium aluminate having the desired properties.
A calcium aluminate obtained by mixing powders of crystalline calcium aluminate and amorphous calcium aluminate in given proportions is known for example from JP2014129203.
However, the final calcium aluminate obtained via such a process comprises fewer amorphous phase than the calcium aluminates obtained chemically, and its properties are therefore different from those of the amorphous calcium aluminates chemically obtained described above.
In particular, the measurement of the C/A molar ratio of calcium aluminate powders obtained through this method does not correspond to the amount of lime and alumina which really goes into solution upon contacting these powders with water. As a consequence it is not possible to predict the C/A molar ratio of such compounds.
Thus, there is a real need for developing a new composition comprising an amorphous calcium aluminate with a C/A ratio comprised within a predetermined range, and having at least the same reactive properties as the amorphous calcium aluminates known to date. Further, there is still a need in the state of the art for providing at least one method for making a composition comprising amorphous calcium aluminate for which the desired C/A molar ratio could be chosen and which would be easy and fast to implement.
It is thus one of the objectives of the present invention to provide a new composition comprising amorphous calcium aluminate as well as a new method for making such composition, while avoiding, at least in part, the previously mentioned drawbacks.